The invention discloses a waste plastic recycling and constant-temperature and constant-pressure granulating device, and belongs to the technical field of waste plastic recycling. The waste plastic recycling and constant-temperature and constant-pressure granulating device comprises a granulating device body, a granulating device shell, a cutter assembly, an air cylinder and a motor, wherein the cutter assembly comprises a cutter bit, a hilt and a cutter holder; a temperature detector is arranged on the inner side of the cutter bit and is used for monitoring the temperature of the cutter bit; a temperature compensating channel is formed in the cavity of the hilt and communicates with a temperature compensator; the temperature compensator is electrically connected with the temperature detector; a control circuit is arranged in the temperature compensator; when the temperature of the cutter bit is in a set temperature range, the control circuit closes the temperature compensator, and on the contrary, the control circuit starts the temperature compensator; a pressure detector is arranged in the granulating device shell; an automatic pressure release valve is arranged on the granulating device shell; and the pressure detector controls opening or closing of the automatic pressure release valve. The waste plastic recycling and constant-temperature and constant-pressure granulating device is good in granulating effect, even in sizes of granules and high in yield, and service lives of the cutter bit and a granulating die head can be prolonged.